Post by thread:Current to voltage- anyone know of an IC?

Hennie Swanepoel wrote: > > I am thinking of using a microcontroller (perhaps a PIC) in an industrial > project. > The controller would have to interface with 0 to 20mA analogue input and > output signals. Does anybody know of an IC(s) that does current-to-voltage > conversion and vica versa, perhaps with the use of one or two passive > components? I am looking for alternatives to using opamps. > > Any suggestions/comments/advice appreciated. > > Hennie Swanepoel You probably mean 4mA to 20mA, don't you? A 250 ohm resistor gives you 1V to 5V. Even though the 250 ohm is not standard E12, you can still get it in very low tolerances (0.1%). -- Friendly Regards Tjaart van der Walt spam_OUTtjaartTakeThisOuTwasp.co.za _____________________________________________________________ | Another sun-deprived R&D Engineer slaving away in a dungeon | | WASP International http://wasp.co.za | | GSM and GPS value-added applications | | Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973 | |_____________________________________________________________|

At 11:36 2/07/97 GMT+200, you wrote: >I am thinking of using a microcontroller (perhaps a PIC) in an industrial > project. >The controller would have to interface with 0 to 20mA analogue input and >output signals. Does anybody know of an IC(s) that does current-to-voltage >conversion and vica versa, perhaps with the use of one or two passive >components? I am looking for alternatives to using opamps. > >Any suggestions/comments/advice appreciated. > >Hennie Swanepoel > Hello Hennie, How about one resistor. V=I*R 50 ohms -> 1 volt full scale reading with 20 mA input. Regards, Ross McKenzie Melbourne Australia

re: 4-20ma Current Loop Transmitter Hennie, Burr-Brown has supplied these kind of ICs for years. Check out their XTR105. You can reach them at: 800-548-6132 http://www.burr-brown.com - Tom At 11:36 AM 7/2/97 GMT+200, you wrote: >I am thinking of using a microcontroller (perhaps a PIC) in an industrial > project. >The controller would have to interface with 0 to 20mA analogue input and >output signals. Does anybody know of an IC(s) that does current-to-voltage >conversion and vica versa, perhaps with the use of one or two passive >components? I am looking for alternatives to using opamps. > >Any suggestions/comments/advice appreciated. > >Hennie Swanepoel

Tjaart wrote: > You probably mean 4mA to 20mA, don't you? A 250 ohm resistor gives you > 1V to 5V. > Even though the 250 ohm is not standard E12, you can still get it in > very low > tolerances (0.1%). No, strangely enough 0 to 20mA is an old Siemens standard used for process control (on at least some of the South-African power stations). I would pefer not to use only a simple resistor (preferably 50 ohm) since the accuracy of (subsequent) A/D conversion would be deteriorate due the quantization effect etc. Also I need to do the opposite: convert a voltage to a current in the 0 to 20 mA range. The usual solution to both these problems involve using opamp circuits. I was hoping that someone had come around to designing IC's dedicated to current/voltage and voltage/current conversion. Regards Hennie

---------- {Quote hidden}> From: Hennie Swanepoel <.....D77190KILLspam@spam@DVPNFS01.ESKOM.CO.ZA> > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: Current to voltage- anyone know of an IC? > Date: Wednesday, July 02, 1997 1:43 PM > > Tjaart wrote: > > > You probably mean 4mA to 20mA, don't you? A 250 ohm resistor gives you > > 1V to 5V. > > Even though the 250 ohm is not standard E12, you can still get it in > > very low > > tolerances (0.1%). > > No, strangely enough 0 to 20mA is an old Siemens standard used for > process control (on at least some of the South-African power > stations). I would pefer not to use only a simple resistor > (preferably 50 ohm) since the accuracy of (subsequent) A/D conversion > would be deteriorate due the quantization effect etc. Also I need to > do the opposite: convert a voltage to a current in the 0 to 20 mA > range. The usual solution to both these problems involve using opamp > circuits. I was hoping that someone had come around to designing > IC's dedicated to current/voltage and voltage/current conversion. > > Regards > Hennie If you are using a micro try looking at the AD420 or AD421 chips from Analog Devices. They both allow you to generate a current from a 16 bit digital value Guy

Hennie Swanepoel wrote: > I was hoping that someone had come around to designing > IC's dedicated to current/voltage and voltage/current conversion. Analog Devices has a series of IC's that handle this. I personally like their stuff better than Burr Brown's. Poke around on the AD ( http://www.analog.com/ ) web site and look for the sensors & sensor interfaces page. They have a nifty 4-20 or 0-20 ma serial dac, and I know they used to have some isolation amps that handle current loop inputs. Somebody out there also has some loop powered isolation amps, but I can't put my finger on who just now. --Tom Rogers VP-R&D Time Tech Inc.

At 19:43 02/07/97 GMT+200, Hennie Swanepoel wrote: >Tjaart wrote: > >> You probably mean 4mA to 20mA, don't you? A 250 ohm resistor gives you >> 1V to 5V. >> Even though the 250 ohm is not standard E12, you can still get it in >> very low >> tolerances (0.1%). > >No, strangely enough 0 to 20mA is an old Siemens standard used for >process control (on at least some of the South-African power >stations). I would pefer not to use only a simple resistor >(preferably 50 ohm) since the accuracy of (subsequent) A/D conversion >would be deteriorate due the quantization effect etc. I'm not sure whether I understand you here. How could a resistor based I-V converter influence the subsequent ADC quantization? Just make sure that the voltage at max current gives you something close to but lower than the max voltage of your converter, and you are at the optimum you can get re quantization. Additionally, _every_ other solution (including opamps and, of course, ready-made modules, which all are opamp based) depends on the same principle (converting a current into a voltage by sending it trough a resistor). No way around it, and rarely a need to use more than one resistor in this case (which is also the optimum in both precision and reliability). If you have any special condition which makes it impossible to use it this way, I'd really be interested to hear what it is. >Also I need to >do the opposite: convert a voltage to a current in the 0 to 20 mA >range. The usual solution to both these problems involve using opamp >circuits. I was hoping that someone had come around to designing >IC's dedicated to current/voltage and voltage/current conversion. An opamp is just this... you need only a few external resistors to do either conversion (I->U 2-3, U->I 4 Rs), and all integrated modules I've seen so far which do this are bigger than an opamp with some resistors. (You can get down to a SOT-223 and 2-4 SMD resistors...) And they, of course, are all based on opamps with a few resistors integrated in the same package; you probably still need the one or other resistor to set the input range. But really, I don't quite see how you can get it smaller and cheaper and easier than with opamps. ------------------------------------- Gerhard Fiedler <.....gerhardKILLspam.....pobox.com> S‹o Paulo - Brazil

Gerhard, _Every_ other solution does not depend on converting the current into a voltage by sending it through a resistor. There are ways "around it." One method is to measure the electric field around the wire; this is the approach in noninvasive procedures. Other way is to dump the current into a capacitor and integrate. This too eliminates the resistor, and is great for applications that require small current measurements as it avoids large resistance values. Burr-brown makes a couple of switched trans-impedance amplifiers for this purpose. Cheers, -jeff ---------- From: Gerhard Fiedler <EraseMEGerhardspam_OUTTakeThisOuTpobox.com> To: PICLISTspam_OUTmitvma.mit.edu Subject: Re: Current to voltage- anyone know of an IC? Date: Friday, July 04, 1997 9:12 AM Additionally, _every_ other solution (including opamps and, of course, ready-made modules, which all are opamp based) depends on the same principle (converting a current into a voltage by sending it trough a resistor). No way around it, and rarely a need to use more than one resistor in this case (which is also the optimum in both precision and reliability).

>I am thinking of using a microcontroller (perhaps a PIC) in an industrial > project. >The controller would have to interface with 0 to 20mA analogue input and >output signals. Does anybody know of an IC(s) that does current-to-voltage >conversion and vica versa, perhaps with the use of one or two passive >components? I am looking for alternatives to using opamps. > >Any suggestions/comments/advice appreciated. > >Hennie Swanepoel > An idea for current to voltage : use LM3900 or equivalent. It's a current input - voltage output quad OP amp single supply. Dorin --------------------------------- Dorin Dogaroiu Research Institute for Computers Bucarest, Romania Email : @spam@dogaroiudKILLspampcnet.pcnet.ro KILLspamddorinKILLspamkappa.ro --------------------------------- >

At 18:29 04/07/97 -0700, Jeff Dickson wrote: >_Every_ other solution does not depend on converting the current into a >voltage by sending it through a resistor. There are ways "around it." One >method is to measure the electric field around the wire; this is the >approach in noninvasive procedures. Other way is to dump the current into >a capacitor and integrate. This too eliminates the resistor, and is great >for applications that require small current measurements as it avoids large >resistance values. Burr-brown makes a couple of switched trans-impedance >amplifiers for this purpose. You're of course right in a general sense, but the original message was talking about a 20mA loop, which is designed to feed into a resistor; there's usually no need for a noninvasive measurement, and the currents usually are not that small as to making it difficult to transform them into a voltage easy to measure. And with the cap charging method, just as with the electric field around the wire (like a hall effect sensor), you finally get a voltage output -- which then has to be fed into an ADC, which usually uses some resistors (amd maybe even opamps) in the analog signal path. I just didn't see the point why trying to avoid a resistor in the signal path leading to the ADC or trying to avoid opamps to generate the 20mA output current (always taking into account that we're talking about a 20mA current loop designed for load resistances in the range of probably up to around 1k). BTW, how do you think that one adjusts the amplification of a trans-imp amp? Most solutions I've seen use resistors, and so you just move the point I was talking about (using resistors to transform current into voltage) >from one place (at the input) to another (maybe in the feedback loop). Which sometimes is of course _very_ appropriate (that's why they make them :-), but again remembering the 20mA current loop... And regarding precision, reliability and price I still have to see a better solution than a resistor (for feeding an ADC) for measuring a 20mA loop. ------------------------------------- Gerhard Fiedler <RemoveMEgerhardTakeThisOuTpobox.com> S‹o Paulo - Brazil